netdev - Re: unintended ipv4 broadcast policy change

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Message-Id: <20110622.194158.820637934689036237.davem@davemloft.net>
Date:	Wed, 22 Jun 2011 19:41:58 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.hengli.com.au
Cc:	netdev@...r.kernel.org
Subject: Re: unintended ipv4 broadcast policy change

From: Herbert Xu <herbert@...dor.hengli.com.au>
Date: Thu, 23 Jun 2011 08:41:34 +0800

> On Wed, Jun 22, 2011 at 04:39:35PM -0700, David Miller wrote:
>> This subtle new behavior is interesting because it means that
>> a DHCP client could be implemented entirely with plain UDP
>> sockets.
> 
> Yes this is indeed possible.  However, for compatibility purposes
> I'm not sure whether we can safely rely on this new behaviour.
> Maybe if we add the disable_ipv4 sysctl we can use it to signal
> the presence of this new behaviour.

The easiest thing to do is to have the DHCP server first go:

	ip addr add 0.0.0.0/0 broadcast 255.255.255.255 dev $(DEVICE)

and in fact this is essentially what ISC DHCP does on NetBSD and
similar.
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